forms because of the expension of the epiblast layer

An amnioscope is a form of endoscope used to examine the amniotic cavity and its foetus.

The amniotic sac is the sac in which the fetus develops in amniotes. Its wall is the amnion, the inner of the two fetal membranes. It encloses the amniotic cavity and the embryo.

Amnion is the membrane that covers the embryo. Chorion is the membrane that covers the amnion, the yolk sac, and the allantois. The space delimited by the chorion and the amnion is called amniotic cavity and it is filled with aminiotic fluid. The amniotic cavity has the functions of preventing desiccation of the embryo and of protecting it against mechanical shocks.

In the first developmental stage of the embryo, you have the ball of cells. In this ball of cells you have cavity filled with fluid called amniotic fluid. This fluid sac separates the embryo, from the outer non-embryo cells. Later on it encloses the fetus in amniotic fluid to give amniotic sac. This sac is attached to uterus from inside.